A polycarbonate is excellent in heat resistance, impact resistance and transparency, so that, in recent years, it has been widely used in many fields.
In the preparation process of the polycarbonate, many investigations have heretofore been done. Among these, a polycarbonate derived from an aromatic dihydroxy compound, for example, 2,2-bis(4-hydroxyphenyl)propane (hereinbelow, referred to as “bisphenol A”) has been industrially produced by any preparation processes of the interfacial polymerization method or the melt polymerization method.
According to the interfacial polymerization method, the polycarbonate is produced from bisphenol A and phosgene, but poisonous phosgene must be used. Also, there remain the problems that the apparatus is corroded by a chlorine-containing compound such as by-produced hydrogen chloride or sodium chloride, and methylene chloride used as the solvent with a large amount, etc., removal of the impurities such as sodium chloride, and remaining methylene chloride, which cause effects on the polymer physical property is difficult, and a large amount of wastewater is generated so that treatment of the wastewater becomes the problem.
On the other hand, as a process for preparing a polycarbonate from an aromatic dihydroxy compound and a diarylcarbonate, for example, it has been known a melt polymerization method from long ago in which bisphenol A and diphenylcarbonate are polymerized in a melt state by transesterification, while removing the by-produced aromatic monohydroxy compound. The melt polymerization method has merits that it does not use a solvent, etc., different from the interfacial polymerization method, but it has an essential problem that a polymer viscosity in the system abruptly increases as the polymerization proceeds and it becomes difficult to remove the by-produced aromatic monohydroxy compound out of the system with good efficiency, whereby the reaction rate is extremely lowered and the polymerization degree is difficultly increased.
To solve the problem, various devices have been investigated for extracting the aromatic monohydroxy compound from the polymer with a high viscosity state. For example, Patent Document 1 (JP Sho. 50-19600B) discloses a screw type polymerization apparatus having a vent portion. Further, Patent Document 2 (JP H2-153923A) discloses a method which uses a thin film evaporation device and a horizontal type polymerization device in combination.
Also, Patent Document 3 (U.S. Pat. No. 5,521,275B) discloses a method in which conversion of a molecular weight of an aromatic polycarbonate is carried out in the presence of a catalyst, by using an extruder having a polymer sealing portion and a vent portion under reduced pressure conditions.
However, according to the methods disclosed in these publications, the molecular weight of the polycarbonate cannot sufficiently be increased. When higher polymerization is carried out by the above-mentioned method using a large amount of a catalyst or under severe conditions in which high shearing is given, whereby bad effects are exerted to the resin such as deterioration of hue of the resin or progress of the cross-linking reaction.
Further, it has been known that a polymerization degree of the polycarbonate can be heightened in the melt polymerization method by adding a polymerization promoter to the reaction system. When the molecular weight can be increased with a short reaction detention time and at a low reaction temperature, production quantity of the polycarbonate can be heightened, and consequently a design of a reactor which is simple and inexpensive can be easily carried out.
Patent Document 4 (EP 0595608 B1) discloses a method of reacting some diarylcarbonates at the time of molecular weight conversion, but significant increase in the molecular weight cannot be accomplished. Also, Patent Document 5 (U.S. Pat. No. 5,696,222B) discloses a method for producing a polycarbonate having a higher polymerization degree by addition of a certain kind of a polymerization promoter, for example, an aryl ester compound of a carbonic acid and dicarboxylic acid including bis(2-methoxyphenyl)carbonate, bis(2-ethoxyphenyl)carbonate, bis(2-chlorophenyl)carbonate, bis(2-methoxyphenyl)terephthalate and bis(2-methoxyphenyl)adipate. The above-mentioned Patent Document 5 teaches that, when an ester compound is used as the polymerization promoter, an ester bond is introduced, and as a result, a polyester carbonate copolymer is formed (in place of a homopolymer), so that hydrolysis stability is low.
Patent Document 6 (JP Patent No. 4112979) discloses a method of reacting some bissalicyl carbonates for increasing the molecular weight of an aromatic polycarbonate.
Patent Document 7 (JP 2008-514754A) discloses a method of highly polymerizing by introducing a polycarbonate oligomer and bissalicyl carbonate, etc., into an extruder.
Patent Document 8 (JP Patent No. 4286914) discloses a method in which a terminal hydroxyl group amount is increased by an active hydrogen compound (dihydroxy compound), and then, coupling of the aromatic polycarbonate having the increased terminal hydroxyl group amount is carried out with a salicylic acid ester derivative.
However, the method disclosed in the above-mentioned publication in which the terminal hydroxyl group of the polycarbonate is required to be increased, requires a reaction step with an active hydrogen compound and a reaction step with a salicylic acid ester derivative so that the steps are complicated, and a polycarbonate having many terminal hydroxyl groups has low thermal stability, so that it has a risk of lowering in physical property. Also, increase in an amount of the hydroxyl group by the active hydrogen compound leads a partial chain dividing reaction, which is accompanied by enlargement of a molecular weight distribution. Further, to obtain a sufficient reaction rate, it is necessary to use a catalyst with a relatively large amount, so that it can be considered the possibility that lowering in physical property at the time of molding is caused.
Some proposals have been made about a process for producing a polycarbonate by adding a diol compound to the reaction system. For example, Patent Document 9 (JP H6-94501B) discloses a preparation process of a high molecular weight polycarbonate by introducing 1,4-cyclohexanediol. However, according to the process disclosed therein, 1,4-cyclohexanediol is added with an aromatic dihydroxy compound from the start of the polycondensation reaction system, 1,4-cyclohexanediol is firstly consumed for the polycarbonating reaction (oligomerization), and thereafter, the aromatic dihydroxy compound is reacted to be highly polymerized. Thus, there are defects that the reaction time becomes relatively long, and physical properties of external appearance such as hue is likely lowered.
Patent Document 10 (JP 2009-102536A) discloses a preparation process of a polycarbonate in which a specific aliphatic diol and an ether diol are copolymerized. However, the polycarbonate disclosed therein has an isosorbide skeleton as a main structure, so that excellent impact resistance required for the aromatic polycarbonate cannot be shown.
Further, there have been proposed a method in which a cyclic carbonate compound is added to a reaction system (Patent Document 11; JP Patent No. 3271353), a method in which a diol having a hydroxyl group basicity of which is higher than that of the dihydroxy compound used is added to a reaction system (Patent Document 12; JP Patent No. 3301453), etc., but in either of the methods, a high molecular weight polycarbonate resin having physical properties sufficiently satisfied cannot be obtained.
Thus, the conventional preparation processes of a high molecular weight aromatic polycarbonate involve many problems, and demands for obtaining an improved preparation process which can retain good qualities of the inherent polycarbonate, and can accomplish sufficiently higher polymerization, still exists.
The present inventors have previously found out a novel process, as a process for obtaining an aromatic polycarbonate which can accomplish a rapid polymerization rate and gives good quality, in which end-capped terminals of the aromatic polycarbonate is connected with an aliphatic diol compound to elongate the chain (Patent Document 13; WO 2011/062220A pamphlet). According to this process, a end-capped terminal of the aromatic polycarbonate is linked to the aliphatic diol compound to elongate the chain, whereby an aromatic polycarbonate resin with a high polymerization degree having an Mw of about 30,000 to 100,000 can be produced within a short period of time. This process produces a polycarbonate with a high speed polymerization reaction, so that branching or cross-linking reaction caused by thermal detention for a long period of time, etc., can be restrained, and deterioration of the resin such as hue, etc., can be avoided.
The present inventors have also previously proposed a preparation process of a branched aromatic polycarbonate resin having a desired branching degree which includes a step of subjecting an aromatic polycarbonate prepolymer into which a branched structure has been introduced and an aliphatic diol compound to transesterification reaction under reduced pressure conditions in the presence of a transesterification catalyst (Patent Document 14; WO 2012/108510A pamphlet).
According to the process for making the highly polymerized polycarbonate by using a linking agent comprising these aliphatic diol compounds, a polycarbonate resin which retains good qualities of the inherent polycarbonate, and accomplishes sufficiently higher polymerization can be produced easily and rapidly, but development of a high molecular weight polycarbonate resin having better thermal stability has been desired.
As one of the factors which impair the thermal stability, a heterologous structure existing in the polycarbonate resin can be mentioned. In the polycarbonate resin obtained by using the melt polymerization method, it has already been known the problem that not a little heterologous structure is present in the main chain, but it is not easy to produce a polycarbonate resin with a little ratio of the heterologous structure by the melt polymerization method. And many contrivances to improve melt characteristics or moldability have been proposed by grasping the existence of the heterologous structure rather positively (Patent Documents 15 to 23).
It is important for the development of a high molecular weight polycarbonate resin with good thermal stability by using the melt polymerization method to produce a polycarbonate resin which has a high molecular weight but a ratio of the heterologous structure is extremely little, but a sufficiently satisfied method has not been proposed.